He was the younger brother of King Soseong, and served as regent during the reign of Aejang.

In 790, Heondeok traveled to Tang China where he distinguished himself and received a high position.

He also sent his son Kim Hŏnjang to Tang with gold and silver Buddhist images to pray for the emperor's eternal peace.

In 824, troubled by threats from the north, the king ordered a 300-ri-long wall built near the Taedong River, which was then the country's northern border.

He is believed to be buried in the Tomb of King Heondeok in Gyeongju, South Korea.